1. Field of the Invention
The present invention relates generally to multi-layer resist systems for use in ion beam and electron beam lithography. More particularly, the present invention relates to a new polysiloxane, specifically a polysilsesquioxane, imaging resist layer which is suitable for use at relatively high temperatures up to approximately 250.degree. C.
2. Description of the Background Art
Advances in the fabrication of integrated circuits and other semi-conductor systems have made it necessary to provide lithography techniques capable of producing pattern sizes in the micrometer and submicrometer range. Electron beam and ion beam lithographic techniques are commonly used because of the extremely fine resolution provided by the electron and/or ion beam. Single layer resist systems have been used with only limited success for electron beam and ion beam imaging. In single layer resist systems one can achieve high aspect ratios in narrow isolated lines with electron-beam lithography. For example, 1 Mm high lines that are 0.2 micron wide are possible. However, one cannot get two lines close together when using a single layer resist with electronbeam lithography. With a single layer resist system, two lines that are 1 micron apart cannot be resolved due to limitations caused by proximity effects. These effects include electron scattering within the resist as well as back scatter of electrons from the substrate. With focused ion beam lithography the proximity effects are much less pronounced; however, the stopping distance of the metal ions used, i.e., silicon, gallium, etc. limits the depth of images to on the order of 0.3 micron or less.
In order to overcome the problems presented by single-layer resist systems, multiple-layer systems have been developed. These systems are classified by the number of layers in the system. Bi-level or bi-layer resist systems have been particularly popular due to their simplicity and high resolution characteristics. The standard bi-layer resist typically includes a relatively thick (approximately 2 micron) organic layer which is generally made from a hard-baked organic polymer chosen for adhesion to the substrate and high resistance to dry etching. This bottom layer is commonly referred to as the planarizing layer. On top of the planarizing layer is placed a relatively thin layer of resist which has the dual function of both an imaging layer and a conformable mask for etching the underlying planarizing layer.
Silicon-containing polymers such as polysiloxanes, silicone oils, silicone gums and silicon-containing methacrylate polymers have been used as the top imaging resist layer. The silicon compounds are radiation sensitive so that they function as a negative resist when exposed to an electron or ion beam. After exposure, the imaging layer is treated with a suitable solvent to remove the unexposed portions of the imaging layer. The exposed portions are insoluble in the solvent due to cross-linking resulting from exposure to the ion beam or electron beam radiation. The remaining imaging resist layer pattern serves as an etch mask for pattern transfer to the underlying planarizing polymer by way of reactive ion etching (RIE) with oxygen plasma. During the processes, the outermost portions of the imaging resist are converted to silicon dioxide. The converted silicon dioxide is believed to serve as the actual etch mask.
Polysiloxanes which are prepared by the hydrolysis of alkyltrichlorosilane have been used as the imaging resist layer. The polysiloxanes are glassy polymers which have a structure characterized as a "broken ladder". The polysiloxanes are high in silicon content so that they provide an especially suitable silicon dioxide mass which has high resistance to RIE in an oxygen plasma. Further, the polysiloxanes have a higher glass transition temperature than silicone oils and gums so that images are not as easily deformed.
Although polysiloxanes have been found useful as an imaging resist layer in bi-layer resist systems, problems have been experienced with cross-linking of polysiloxane at temperatures above 70.degree. to 100.degree. C. Many times, it is desirable to carry out ion beam or electron beam lithography on a resist that has been heated at temperatures above 100.degree. C. to improve the adhesion of the resist to the substrate and to eliminate as much solvent from the resist as possible. Cross-linking of the polysiloxane at temperatures above 100.degree. C. results in the formation of an imaging resist layer which is insoluble over its entire surface and therefore unsuitable for electron beam and ion beam lithography. It would be desirable to provide an improved polysiloxane imaging resist which can be used for electron beam or ion beam lithography at temperatures above 100.degree. C. without undesirable cross-linking of the polymer.